The present invention is generally directed to processes for the preparation of toner resins and toner compositions. More specifically, the present invention in embodiments relates to melt mixing processes, batch or continuous, such as, for example, reactive extrusion for preparing crosslinked toner resins. The present invention in embodiments relates to processes for crosslinking reactive linear resins that can be selected for the preparation of crosslinked toner resins and which resins can be utilized as heat fixable toners with superior fusing and vinyl offset performance, and which processes induce or enable initiator decomposition by reaction with an amine functionalized polyester. The process of the present invention in embodiments enable process flexibility, safe reactions with high temperature initiators, such as t-butylhydroperoxide, and crosslinking of the resin becomes primarily a function of mixing during extrusion, and not a function of both temperature and mixing. In embodiments the present invention is directed to improved processes for the preparation of crosslinked toner resins, and toner compositions thereof, reference U.S. Ser. No. 814,641 (D/91117) and U.S. Pat. No. 5,227,460 (D/91117Q), the disclosures of which are totally incorporated herein by reference, by the reaction, preferably in an extruder, of an initiator with an amine polyester, and the subsequent reaction with a resin that contains at least one unsaturation site, or double bond, including polyesters like SPAR.TM., whereby crosslinking of the resin is accomplished. In embodiments, a resin, such as a polyester, like a SPAR.TM. polyester, an amine functionalized polyester, and an initiator are added to an extruder, followed by heating whereby the initiator reacts with the amine functionally to form free radicals, which subsequently react with the unsaturation sites of the resin to form a crosslinked resin. The aforementioned extrusion reaction can be accomplished in the presence of pigments like carbon black, and toner additives, such as charge additives to enable toner compositions.
Toner utilized in development in the electrographic process may be generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known, including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. Examples of colorants include carbon black and magnetites, and as charge enhancing additive examples there may be selected alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfates, and the like.
To fix the toner to a support medium, such as a sheet of paper or transparency, hot roll fixing is commonly used. In this method, the support medium with a toner image is transported between a heated fuser roll and a pressure roll with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image. This fixing system is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is referred to as the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images resulting, for example, in blurred images. This undesirable phenomenon is known as offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is referred to as the fusing latitude.
The hot roll fixing system and a number of toners used therein may, however, exhibit several problems, for example the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fu; and fuser roll bearings. Also, offsetting can be a problem. Furthermore, toners containing vinyl type binder resins, such as styrene-acrylic resins, may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image is contacted, for a period of time, with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface.
There is a need for a toner resin with low fixing temperatures and high offset temperatures or wide fusing latitude, and superior vinyl offset properties, and process for the preparation of such a resin with excellent process latitudes; and wherein crosslinking of the resin is primarily a function of mixing during the reactive extrusion process.
To prepare lower fix temperature resins for toner, the molecular weight of the resin may be lowered. Low molecular weight amorphous polyester resins and epoxy resins have been selected to prepare low temperature fixing toners. For example, toners utilizing polyester resins as a binder are disclosed in U.S. Pat. Nos. 3,590,000 and 3,681,106. The minimum fixing temperature of polyester binder resins can be rendered lower than that of other materials, such as styrene-acrylic resins. However, this may result in the lowering of the hot offset temperature and, as a result, decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
To prevent fuser roll offsetting and to increase fusing latitude of toners, modification of the binder resin structure by conventional polymerization processes, for example by branching, crosslinking, and the like has been attempted. For example, in U.S. Pat. No. 3,681,106 a process is disclosed whereby a polyester was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a polyol, especially a trivalent polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is by crosslinking during polymerization. In, for example, U.S. Pat. No. 3,941,898, a crosslinked vinyl type polymer prepared using conventional crosslinking was used as the binder resin. Similar disclosures for vinyl type resins are presented in U.S. Pat. Re. No. 31,072 (a reissue of No. 3,938,992); Nos. 4,556,624; 4,604,338 and No. 4,824,750. Also, crosslinked polyester binder resins using conventional polycondensation processes improving offset resistance are known, reference for example U.S. Pat. No. 3,681,106.
While improvements can be obtained in offset resistance and entanglement resistance, a major drawback may ensue with crosslinked resins prepared by conventional polymerization, both vinyl type processes including solution, bulk, suspension and emulsion polymerizations and polycondensation processes. In these processes, monomer and crosslinking agent are added to the reactor. The crosslinking reaction is not considered very rapid and chains can grow in more than two directions at the crosslinking point by the addition of monomers. Three types of polymer configurations may thus be produced, a linear and soluble portion referred to as the linear portion, a crosslinked portion which is low in crosslinking density and, therefore, is soluble in some solvents, such as tetrahydrofuran, toluene and the like, and is referred to as the sol; and a portion comprising highly crosslinked gel particles which is not soluble in substantially any solvent, such as tetrahydrofun, toluene and the like, and is referred to as the gel. The second portion with low crosslinking density (sol) is primarily responsible for widening the molecular weight distribution of the soluble part which results in an elevation of the minimum fixing temperature of the toner. Also, a drawback of these processes is that as more crosslinking agent is used the gel particles or very highly crosslinked insoluble polymer with high molecular weight increases in size. The large gels can be more difficult to disperse pigment in causing unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, with vinyl polymers the toners produced often evidence vinyl offset.
In U.S. Pat. No. 4,533,614, there is disclosed a process for preparing loosened crosslinked polyester binder resin which showed low temperature fix and good offset resistance, and wherein metal compounds were selected as crosslinking agents. Similar teachings are presented in U.S. Pat. No. 3,681,106, and Japanese Laid-open Patent Applications Nos. 94362/1981, 116041/1981 and 166651/1980. More specifically, there is disclosed in the No. '614 patent that the incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner. Also, with color toners other than black, for example cyan toners, metal complexes can adversely the color of the pigments. It is also known that metal containing toner can have disposal problems in some areas, such as for example in the State of California, U.S.A. Metal complexes are often also expensive materials.
In U.S. Pat. No. 4,894,308, and U.S. Pat. No. 4,973,439 there are disclosed, for example, extrusion processes for preparing electrophotographic toner compositions in which pigment and charge control additive were dispersed into the binder resin in the extruder. However, in these patents, there is no suggestion of a chemical reaction occurring.
An injection molding process for producing crosslinked synthetic resin molded articles is disclosed in U.S. Pat. No. 3,876,736 in which polyolefin or polyvinyl chloride resin and crosslinking agent were mixed in an extruder, and then introduced into an externally heated reaction chamber outside the extruder wherein the crosslinking reaction occurred at increased temperature and pressure, and at low or zero shear.
In U.S. Pat. No. 4,089,917, an injection molding and crosslinking process is disclosed in which polyethylene resin and crosslinking agent were mixed in an extruder, and reacted in reaction chambers at elevated temperature and pressure. Heating of the resin mixture occurred partially by high shear in inlet flow orifices. However, the crosslinking reaction still took place in the reaction chambers at low or zero shear, and the final product is a thermoset molded part, and thus, is not useful for toner resins.
A process for dispensing premixed reactive precursor polymer mixtures through a die for the purposes of reaction injection molding or coating is described in U.S. Pat. No. 4,990,293 in which polyurethane precursor systems were crosslinked in the die and not in the extruder. The dimensions of the die channel were determined such that the value of the wall shear stress was greater than a critical value in order to prevent gel buildup and consequent plugging of the die. The final product is a thermoset molded part, and thus, is not useful for toner resins.
The processes disclosed in U.S. Pat. Nos. 3,876,736; 4,089,917 and 4,990,293 are not reactive extrusion processes because the crosslinking in each situation occurs in a die or a mold, and not in an extruder. These processes are for producing engineering plastics, such as thermoset materials which cannot, it is believed, be remelted once molded, and thus are not suitable for toner applications.
Illustrated in copending patent application U.S. Ser. No. 814,641 (D/91117) U.S. Pat. No. 5,227,460 (D/91117Q), the disclosures of which are totally incorporated herein by reference, is, for example, a reactive melt mixing process of preparing low fix temperature toner resin comprising the steps of (a) melting a reactive base resin, thereby forming a polymer melt; and (b) crosslinking said polymer melt under high shear to form a crosslinked toner resin and in embodiments further comprising the step of mixing a chemical initiator into the polymer melt at a temperature lower than the onset of crosslinking temperature, thereby producing a dispersion of the chemical initiator in the polymer melt prior to onset of crosslinking of the polymer melt. Disadvantages that may be associated with the aforementioned processes of the copending applications in embodiments thereof include decomposition of the initiator at high temperatures to form free radicals which in turn react prematurely, that is before complete mixing of initiator and polymer occurs, with the double bond of the polyester resin selected to form crosslinked sites, and heating and thorough mixing are needed. High levels of initiator may be necessary in the processes illustrated in the copending applications, and typically the initiators selected have a high explosion hazard relative to the preferred initiators selected for the processes of the present invention. These and other disadvantages are avoided or minimized with the processes of the present invention.